In general, an air supply unit comprises a manifold having an air inlet orifice and a plurality of supply ducts, which are in communication with the manifold and have respective outlet orifices, each of which is capable of supplying a respective cylinder of the internal combustion engine. Some supply units are made with supply ducts that have variable geometry. In this specific case, each supply duct communicates with the manifold through a first and a second opening, which are arranged in different zones of the supply duct in such a manner as to define a first complete path of air along the supply duct, which path is delimited by the first opening and the outlet orifice, and a second shorter path that extends between the second opening and the outlet orifice. The second path is thus part of the first path. Moreover, a valve for selectively opening and closing the second opening is fitted in correspondence with second opening. The unit of this configuration has the function of selecting the preferred path depending upon the operation of the endothermic engine: when the rotational speed of the endothermic engine is low and high torque is required, it is preferable for the path followed by the combustion air to be long and the valve is thus in the closed position; when the rotational speed of the endothermic engine is high and high power is required, it is preferable for the path followed by the air to be short and the valve is thus in the open position. From a functional standpoint, the valve of each supply duct is actuated depending on the rotational speed of the endothermic engine.
The unit described above has a number of disadvantages: the first disadvantage is that fitting a valve in each second opening is a complex and costly operation; the second disadvantage is that the unit has no valve in correspondence with the first opening and, although the combustion air is preferably channelled over the second, shorter, path, a certain quantity of combustion air passes through the first path, so reducing the advantage of providing supply ducts having variable geometry.